The Catachan Devil is a creature native to the Death World Catachan, but has been introduced into several other worlds. Devils are scorpion-like in appearance, with a series of segmented body sections carrying pairs of legs and a terminus in a long tail with a poisonous barb, which curves forward over the creature's back. As Catachan Devils grow, they develop more and more mid-sections and pairs of legs. A fully grown adult can have up to thirty sets of legs, and can reach up to thirty meters in length.3

The front legs of older male Devils become grossly enlarged, resembling scorpion claws. These mighty beasts are often dubbed 'Fiddlers' in Catachan slang, and are quite rightly feared for their ferocity and raw power.

They are able to move with surprising speed for something of their size, being superbly adapted to move across a large variety of terrain. Once capturing a victim, the Devil will shred it with razor claws or, alternatively, will sting into insensibility with barbs located around its maw. The victim will then be consumed.

Catachan Devils are territorial, yet quite sociable creatures, tending to form into nesting groups. These groups often contain several large Devils, as well as a few dozen or so of 'Devilspawn' (younger Devils). The entire nest will come together to defend their territory from any intruders, be it rival Devils, colonists, or other predators, and they have even been known to attack Imperial Guard Tank Companies.4

It is also rumoured that Devils are the descendants of a long-lost Tyranid Vanguard organism that has effectively become feral.

That photo is actually incorrect. All tau weapons or battle suits would have some sort of DNA identification system. Like in a book my dad read (not quite sure which book) it said that an IG or some other foreign soldier, jumped into a tau battlesuit and the hatch closed and reopened and the guy was fried burnt to death.